1. Field of the Invention
The present invention relates to a resonance device in which a transmission line such as a micro-strip line or a coplanar line is coupled to a resonator. In addition, the invention relates to an oscillator, a filter, a duplexer, and a communication device incorporating the same.
2. Description of the Related Art
A conventional resonance device will be illustrated referring to FIG. 12. This figure is a perspective view of the conventional resonance device.
The conventional resonance device 110 shown in FIG. 12 is constituted of a micro-strip line 120 as a transmission line and a resonator 111. The micro-strip line 120 is composed of a dielectric substrate 121, a main conductor 122 formed on the upper surface thereof and an earth conductor 123 formed on the lower surface thereof. The resonator 111 is a cylindrical dielectric member, a part of which is arranged over the main conductor 122 of the micro-strip line 120. In the resonance device 110 having such a structure, an electromagnetic field is excited surrounding the micro-strip line 120 by current flowing through the main conductor 122 of the micro-strip line 120. As a result, the electromagnetic field excited by the current is coupled to the resonator 111 so that the resonator 111 resonates in a TE01xcex4 mode.
In general, when a resonance device is used to form an oscillator or a filter, a part of the characteristics of the oscillator or the filter depends on the strength of the coupling between a transmission line and a resonator used in the resonance device. For example, the stronger the coupling between the transmission line and the resonator, the greater the oscillating output of the oscillator, and the wider the band width characteristics of the filter.
In such a conventional resonance device, however, coupling beyond a certain level of strength cannot be obtained due to the dispersive characteristics of a micro-strip line, which will be described below. The dispersive characteristics of a micro-strip line are also described in xe2x80x9cMicrowave Planar Passive Circuits and Filters,xe2x80x9d by J. Helszajn, John Wiley and Sons, 1994, pp 90-93, and other publications. Thus, when an oscillator having a large output and a-filter having wide frequency bandwidth characteristics are desired, since it is impossible to make the coupling between the transmission line and the resonator stronger than a certain level, there is a problem in that an oscillator and a filter having such desired characteristics cannot be obtained.
Referring to FIG. 13, a description will be given of the problem. FIG. 13 is a graph showing the result of a simulation about the reflection characteristics of a resonance device with respect to a frequency. In this figure, reference numerals S11 indicates the value of reflection characteristics, which is a ratio of output-signal strength/input-signal strength obtained when a signal is input from one side of a micro-strip line shown in FIG. 12 and an output signal is observed on the same side.
The resonance device used in the simulation has a structure shown in FIG. 12, in which the relative permittivity of the dielectric substrate 121 of the micro-strip line 120 is set to be 3.2, the thickness of the dielectric substrate 121 is set to be 0.3 mm, and the line width of the main conductor 122 is set to be 0.72 mm. In addition, the relative permittivity of the resonator 111 is set to be 24, the diameter thereof is set to be 2.0 mm, and the thickness thereof is set to be 0.8 mm. As indicated by the graph shown in FIG. 13, in the conventional resonance device 110, the reflection characteristics is 3 dB when the resonating frequency is 28.5 GHz. In other words, this shows a fact that in the case of such a conventional resonance device, many signals pass through without being reflected at a resonance frequency, with an implication that coupling between the micro-strip line 12 and the resonator 111 in the resonance device 110 is weak.
A description will be given below about the reason why the coupling between the transmission line and the resonator in the conventional resonance device is weak. This is a case in which a micro-strip line is used as the transmission line.
In general, in a micro-strip line, it is ideal that an electromagnetic field excited by current flowing through a main conductor all exists on a surface vertical to a signal-propagating direction. However, in fact, an electromagnetic field is distributed both in an air space around the micro-strip line and in a dielectric substrate. Since the permittivity of the air space and that of the dielectric substrate are different, a phase velocity of the electromagnetic field is different between the air space and the dielectric substrate. As a result, it is impossible to obtain the ideal situation in which the electromagnetic field all exists on the surface vertical to a signal-propagating direction. That is, in this situation, the electromagnetic field excited by current flowing through the main conductor includes a component parallel to a signal-propagating direction. FIGS. 14A and 14B each show the distribution of the electromagnetic field having the component parallel to a signal-propagating direction. FIG. 14A shows the distribution of an electric field and FIG. 14B shows that of a magnetic field.
According to an equivalent principle, in the conventional resonance device, the electromagnetic field associated with coupling between the resonator and the transmission line is an electromagnetic field in a direction substantially vertical to a signal-propagating direction. In contrast, the electromagnetic field in a direction parallel thereto is not associated with coupling between the resonator and the transmission line. In other words, when the electromagnetic-field component parallel to a signal-propagating direction is increased, it is suggested that this increases an undesired electromagnetic-field component in terms of the coupling between the resonator and the transmission line. Thus, this is a factor that weakens the coupling between them.
Meanwhile, the higher the frequency, the larger the electromagnetic-field component parallel to a signal-propagating direction. This will be described referring to FIG. 15, which shows the relationship between an effective relative permittivity and a frequency. In addition, a micro-strip line used in this situation has a structure shown in FIG. 12, in which the relative permittivity of the dielectric substrate 121 is set to be 3.2, the thickness of the dielectric substrate 121 is set to be 0.3 mm, and the line width of the main conductor 122 is set to be 0.72 mm.
In the micro-strip line shown in FIG. 12, as described above, although the electromagnetic field is distributed both in the air space around the micro-strip line and in the dielectric substrate, the permittivity of the air space is different from that of the dielectric substrate. As a result, energy existing in the air space flows into the dielectric substrate by which distortion occurs in the distributions of the electromagnetic field, with the result that an electromagnetic-field component parallel to a signal-propagating direction is generated. In other words, the higher the proportional amount of energy existing in the dielectric substrate, the larger the electromagnetic field-component parallel to a signal-propagating direction, which weakens coupling between the resonator and the transmission line.
Next, a description will be given of the relationship between the ratio of the amount of energy existing in the dielectric substrate and an effective relative permittivity. For example, when the relative permittivity of the dielectric substrate is indicated by the symbol er and the ratio between the energy existing in the air space and that in the dielectric substrate is set to 1:1, the effective relative permittivity indicated by the symbol e eff is approximately equal to (1+xcex5r)/2. When the energy existing in the dielectric substrate is increased and the ratio between the energy existing in the air space and that in the dielectric substrate is set to 1:2, the effective relative permittivity xcex5 eff is approximately equal to (1+2xcex5r)/3. In this situation, the value of xcex5 eff is closer to that of er. That is, the increase in the proportional amount of the energy existing in the dielectric substrate is equivalent to how close the effective relative permittivity is to the relative permittivity of the dielectric substrate.
FIG. 15 is a graph showing the relationship between a frequency and an effective relative permittivity. In this figure, at a frequency of 30 GHz, the effective relative permittivity amounts to approximately 90% of the permittivity 3.2 of the dielectric substrate, and at frequencies over 30 GHz, the effective relative permittivity is closer to the permittivity of the dielectric substrate. Therefore, the higher the frequency is, the closer to the relative permittivity of the dielectric substrate the effective relative permittivity is, and at the same time, the ratio of the amount of energy existing in the dielectric substrate is increased, which leads to an increase in the electromagnetic-field component parallel to a signal-propagating direction. This parallel electromagnetic-field component is not associated with coupling between the resonator and the transmission line.
Recently, in communication equipment, the use of frequencies in a quasi-millimeter wave band or a millimeter wave band has been increasing. The use of high frequencies has become inevitable. However, as described above, there is a problem in that, the higher the frequency, the weaker the coupling between the resonator and the transmission line used in a resonance device.
An additional problem is that, in order to strengthen the coupling between the resonator and the transmission line, the resonator may be disposed close to the main conductor of the transmission line. However, when the amount that the main conductor of the transmission line is inserted into a resonating space is increased, conductor losses are increased, which causes a problem in that an unloaded Q of the resonator is reduced.
Accordingly, the present invention is directed to solving these problems and providing a resonance device capable of strengthening coupling between a resonator and a transmission line without shortening the distance between the resonator and the transmission line, and an oscillator, a filter, a duplexer, and a communication device incorporating the same.
To this end, according to a first aspect of the present invention, there is provided a resonance device including a transmission line formed by a dielectric substrate, a main conductor, and an earth conductor, both of the conductors being formed on the dielectric substrate, and a resonator disposed in proximity to the main conductor of the transmission line and electromagnetically coupled to the transmission line. In this arrangement, at least one electrodeless portion is formed in a part of the main conductor of the transmission line, the part being coupled to the resonator.
In the resonance device, formation of the electrodeless portion, which is advantageously a slit, permits current flowing in a direction vertical to a signal-propagating direction to be cut off, by which the occurrence of an electromagnetic field in a direction parallel to the signal-propagating direction is suppressed in response to the cut-off of current. As a result, the ratio of the electromagnetic-field component parallel to the signal-propagating direction as an undesired electromagnetic-field component in the coupling between the resonator and the transmission line is reduced, and the ratio of the electromagnetic-field component in a direction vertical thereto is thereby increased, by which the coupling between the resonator and the transmission line can be strengthened. Preferably, the electrodeless portion has the form of a slit and is formed along a direction in which the main conductor of the transmission line extends.
In addition, according to a second aspect of the present invention, there is provided an oscillator including the resonance device described above, a casing containing the resonance device, and a printed circuit board.
Furthermore, according to a third aspect of the present invention, there is provided a communication device including at least one of a transmission circuit and a reception circuit, and an antenna, in which one of the transmission circuit and the reception circuit has an oscillator, which is an oscillator as described above.
Furthermore, according to a fourth aspect of the present invention, there is provided a filter including the resonance device described above and an input/output connector.
Furthermore, a duplexer in accordance with a fifth aspect of the present invention includes at least two filters, input/output connectors for connecting to the filters, and an antenna connector for commonly connecting to the filters. At least one of the filters in the duplexer is a filter as described above.
Furthermore, a communication device in accordance with a sixth aspect of the present invention includes the duplexer described above, a transmission circuit for connecting to at least one input/output connector of the duplexer, a reception circuit for connecting to at least one input/output connector of the duplexer, which is different from that for connecting to the transmission circuit, and an antenna for connecting to the antenna connector of the duplexer.
This arrangement strengthens the coupling between the resonator and the transmission line so as to obtain an oscillator with a large output, a filter with wider band frequency characteristics, and the like.
Other features and advantages of the invention will be understood from the following detailed description of embodiments thereof, with reference to the drawings.